Flat heat pipe and method for manufacturing flat heat pipe

ABSTRACT

An exemplary flat heat pipe includes a hollow, flattened casing and a first wick structure and a second wick structure received in the casing. The casing includes a top plate and a bottom plate opposite to the top plate. The first wick structure is folded by a steel sheet with a plurality of pores, and the second wick structure is made of sintered metal powder. The first and second wick structures are disposed at inner sides of the bottom and top plates of the casing, respectively. The first and second wick structures contact each other. The casing defines two vapor channels at opposite lateral sides of the combined first and second wick structures, respectively.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to two co-pending applications respectivelyentitled “FLAT HEAT PIPE AND METHOD FOR MANUFACTURING THE SAME”(attorney docket number US33318) and “FLAT TYPE HEAT PIPE AND METHOD FORMANUFACTURING THE SAME” (attorney docket number US34501), both assignedto the assignee of this application. The application entitled “FLAT HEATPIPE AND METHOD FOR MANUFACTURING THE SAME” was filed on 2010 Jun. 28.The application entitled “FLAT TYPE HEAT PIPE AND METHOD FORMANUFACTURING THE SAME” is filed on the same date as this application.The two related applications are incorporated herein by reference.

BACKGROUND

1. Technical Field

The disclosure generally relates to heat transfer apparatuses, andparticularly to a flat heat pipe with high heat transfer performance.

2. Description of Related Art

Heat pipes are widely used in various fields for heat dissipationpurposes due to their excellent heat transfer performance. One commonlyused heat pipe includes a sealed tube made of heat conductive material,and a working fluid contained in the sealed tube. The working fluidconveys heat from one end of the tube, typically referred to as anevaporator section, to the other end of the tube, typically referred toas a condenser section. Preferably, a wick structure is provided insidethe heat pipe, lining an inner wall of the tube, and drawing the workingfluid back to the evaporator section after it condenses at the condensersection.

During operation, the evaporator section of the heat pipe maintainsthermal contact with a heat-generating electronic component. The workingfluid at the evaporator section absorbs heat generated by the electroniccomponent, and thereby turns to vapor. Due to the difference in vaporpressure between the two sections of the heat pipe, the generated vapormoves, carrying the heat with it, toward the condenser section. At thecondenser section, the vapor condenses after transferring the heat to,for example, fins thermally contacting the condenser section. The finsthen release the heat into the ambient environment. Due to thedifference in capillary pressure which develops in the wick structurebetween the two sections, the condensate is then drawn back by the wickstructure to the evaporator section where it is again available forevaporation.

Wick structures currently available for heat pipes can be fine groovesdefined in the inner surface of the tube, screen mesh or fiber insertedinto the tube and held against the inner surface of the tube, orsintered powder bonded to the inner surface of the tube by a sinteringprocess. The grooved, screen mesh and fiber wick structures provide ahigh capillary permeability and a low flow resistance for the workingfluid, but have a small capillary force to drive condensed working fluidfrom the condenser section toward the evaporator section of the heatpipe. In addition, a maximum heat transfer rate of these wick structuresdrops significantly after the heat pipe is flattened. The sintered wickstructure provides a high capillary force to drive the condensed workingfluid, and the maximum heat transfer rate does not drop significantlyafter the heat pipe is flattened. However, the sintered wick structureprovides only a low capillary permeability, and has a high flowresistance for the working fluid.

What is needed, therefore, is a flat heat pipe with high capillarypermeability and low flow resistance and a method for manufacturing sucha flat heat pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an abbreviated, lateral side plan view of a flat heat pipe inaccordance with a first embodiment of the disclosure.

FIG. 2 is an enlarged, transverse cross section of the flat heat pipe ofFIG. 1, taken along line II-II thereof.

FIG. 3 is a flowchart of an exemplary method for manufacturing the flatheat pipe of FIG. 1.

FIG. 4 is an abbreviated, isometric view of a cylindrical tube and acylindrical mandrel used for manufacturing the flat heat pipe accordingto the method of FIG. 3.

FIG. 5 is an enlarged, transverse cross section of the cylindricalmandrel of FIG. 4, taken along line V-V thereof.

FIG. 6 is a transverse cross section of a semi-finished flat heat pipemanufactured according to the method of FIG. 3, showing a semi-finishedfirst wick structure and a semi-finished second wick structure in thecylindrical tube of FIG. 4.

FIG. 7 is similar to FIG. 2, but shows a transverse cross section of aflat heat pipe according to a second embodiment of the disclosure.

FIG. 8 is similar to FIG. 2, but shows a transverse cross section of aflat heat pipe according to a third embodiment of the disclosure.

FIG. 9 is a transverse cross section of a semi-finished flat heat pipemanufactured according to a method similar to the method of FIG. 3,showing a semi-finished first wick structure and a semi-finished secondwick structure in the cylindrical tube of FIG. 4, the semi-finished flatheat pipe corresponding to the flat heat pipe of FIG. 8.

FIG. 10 is a transverse cross section of a cylindrical mandrel used formanufacturing the flat heat pipe of FIG. 8 according to the methodsimilar to the method of FIG. 3.

FIG. 11 is similar to FIG. 2, but shows a transverse cross section of aflat heat pipe according to a fourth embodiment of the disclosure.

DETAILED DESCRIPTION

Referring to FIGS. 1-2, a flat heat pipe 10 in accordance with a firstembodiment of the disclosure is shown. The flat heat pipe 10 includes aflat tube-like casing 11 with two ends thereof sealed, and a variety ofelements enclosed in the casing 11. Such elements include a first wickstructure 12, a second wick structure 13, and a working medium (notshown). The flat heat pipe 10 has an evaporator section 101 and anopposite condenser section 102 respectively located at opposite endsthereof along a longitudinal direction thereof.

The casing 11 is made of metal or metal alloy with a high heatconductivity coefficient, such as copper, copper-alloy, or othersuitable material. The casing 11 has a width much larger than itsheight. In particular, the casing 11 has a flattened transverse crosssection. To meet the height requirements of common electronic products,the height of the casing 11 is preferably less than or equal to 2millimeters (mm). The casing 11 is hollow, and longitudinally defines aninner space 110 therein. The casing 11 includes a top plate 111, abottom plate 112 opposite to the top plate 111, and two side plates 113,114 extending between the top and bottom plates 111, 112. The top andbottom plates 111, 112 are flat and parallel to each other. The sideplates 113, 114 are arcuate and respectively disposed at oppositelateral sides of the casing 11.

The first wick structure 12 is elongated, and extends longitudinallythrough the evaporator section 101 and the condenser section 102. Thefirst wick structure 12 is flattened during manufacture of the flat heatpipe 10 to form a generally block-shaped structure. In addition, thefirst wick structure 12 is a multilayer-type structure. Themultilayer-type structure is obtained from an elongated steel sheet witha plurality of pores (not labeled). The elongated steel sheet is foldedon itself again and again (i.e. concertianed), and then layered on thebottom plate 112. Alternatively, the first wick structure 12 can be amonolayer-type steel sheet with a plurality of pores. The first wickstructure 12 provides a large capillary permeability for the workingmedium and has a low flow resistance to the working medium, therebypromoting the flow of the working medium in the flat heat pipe 10.

The first wick structure 12 is disposed at a middle of one inner side ofthe casing 11, with a bottom surface of the first wick structure 12snugly attached to an inner surface of the bottom plate 112 of thecasing 11, and a top surface of the first wick structure 12 snugly incontact with the second wick structure 13.

The second wick structure 13 is made of sintered metal powder such ascopper powder. The second wick structure 13 provides a large capillaryforce to drive condensed working medium at the condenser section 102 toflow toward the evaporator section 101 of the flat heat pipe 10. Inparticular, a maximum heat transfer rate (Q_(max)) of the second wickstructure 13 does not significantly drop after the flat heat pipe 10 isflattened. The second wick structure 13 is disposed at a middle ofanother inner side of the casing 11 opposite to the first wick structure12. In other words, the second wick structure 13 directly faces and isaligned with the first wick structure 11. The second wick structure 13tapers from a top surface thereof farthest away from the first wickstructure 12 toward a bottom side thereof in contact with the first wickstructure 12. In this embodiment, the second wick structure 13 has agenerally triangular prism shape. The top surface of the second wickstructure 13 is attached to an inner surface of the top plate 111 of thecasing 11 by sintering, and the bottom lateral side of the second wickstructure 13 forms a rounded ridge attached to a middle of the topsurface of the first wick structure 12.

The first and second wick structures 12, 13 are stacked together in aheight direction of the casing 11, and divide the inner space 110 of thecasing 11 into two longitudinal vapor channels 118. The vapor channels118 are disposed at opposite lateral sides of the combined first andsecond wick structures 12, 13, respectively, and provide passagesthrough which the vapor flows from the evaporator section 101 to thecondenser section 102.

The working medium is injected into the casing 11 and saturates thefirst and second wick structures 12, 13. The working medium usuallyselected is a liquid such as water, methanol, or alcohol, which has arelatively low boiling point. The casing 11 of the flat heat pipe 10 isevacuated and hermetically sealed after injection of the working medium.The working medium can evaporate when it absorbs heat at the evaporatorsection 101 of the flat heat pipe 10.

In operation, the evaporator section 101 of the flat heat pipe 10 isplaced in thermal contact with a heat source (not shown) that needs tobe cooled. The heat source can, for example, be a central processingunit (CPU) of a computer. The working medium contained in the evaporatorsection 101 of the flat heat pipe 10 vaporizes when it reaches a certaintemperature after absorbing heat generated by the heat source. Thegenerated vapor moves from the evaporator section 101 via the vaporchannels 118 to the condenser section 102. After the vapor releases itsheat and condenses in the condenser section 102, the condensed workingmedium is returned via the first and second wick structures 12, 13 tothe evaporator section 101 of the flat heat pipe 10, where the workingmedium is again available to absorb heat.

In the flat heat pipe 10, the first wick structure 12 is formed byfolding the elongated steel sheet, and is disposed at one inner side(i.e., the inner surface of the bottom plate 112) of the casing 11. Thesecond wick structure 13 is made of sintered metal powder, and isdisposed at another opposite inner side (i.e., the inner surface of thetop plate 111) of the casing 11. The first and second wick structures12, 13 contact each other. Therefore, during operation of the flat heatpipe 10, the working medium can be freely exchanged between the firstand second wick structures 12, 13. Thus, the flat heat pipe 10 has notonly a high capillary permeability and a low flow resistance due to thefirst wick structure 12 being formed by folding the steel sheet, butalso a large capillary force due to the second wick structure 13 beingmade of sintered powder. Thereby, a heat transfer performance of theflat heat pipe 10 is improved.

Table 1 below shows an average of maximum heat transfer rates (Qmax) andan average of heat resistances (Rth) of thirty-five conventionalsintered heat pipes and thirty-five flat heat pipes 10 in accordancewith the present disclosure, all of which have a height of 2 mm. Table 2below shows an average of Qmax and an average of Rth of thirty-fiveconventional sintered heat pipes and thirty-five flat heat pipes 10 inaccordance with the present disclosure, all of which have a height of1.5 mm. Qmax represents the maximum heat transfer rate of each heat pipeat an operational temperature of 50° C. Rth is obtained by dividing thedifference between an average temperature of the evaporator section ofthe heat pipe and an average temperature of the condenser section of theheat pipe by Qmax. A diameter of the transverse cross section (i.e. awidth) and a longitudinal length of each of the conventional sinteredheat pipes are 6 mm and 200 mm, respectively, which are equal to thediameter of the transverse cross section (i.e. the width) and thelongitudinal length of each of the flat heat pipes 10, respectively.Tables 1 and 2 show that the average of Rth of the flat heat pipes 10 issignificantly less than that of the conventional sintered heat pipes,and that the average of Qmax of the flat heat pipes 10 is significantlymore than that of the conventional sintered heat pipes.

TABLE 1 average of Qmax average of Rth Types of heat pipes (unit: W)(unit: ° C./W) Conventional sintered 24.5 0.212 heat pipes Flat heatpipes 10 28.6 0.189

TABLE 2 average of Qmax average of Rth Types of heat pipes (unit: W)(unit: ° C./W) Conventional sintered 15.6 0.356 heat pipes Flat heatpipes 10 22.4 0.253

FIG. 3 summarizes an exemplary method for manufacturing the flat heatpipe 10. The method includes the following steps:

Referring also to FIGS. 4-6, firstly, a mandrel 14, a first wickstructure preform 15 and a tube 16 are provided. The mandrel 14 iselongated and generally cylindrical, and longitudinally defines a notch141 in a circumferential surface thereof. The notch 141 is located at abottom side of the mandrel 14, and spans through both a front endsurface and a rear end surface of the mandrel 14. A transverse crosssection defined by the notch 141 is arc-shaped. A longitudinal top wallportion of the mandrel 14 is horizontally cut, thereby defining a cutout142 in a circumferential surface of the mandrel 14. That is, the cutout142 is located at the top side of the mandrel 14. An inmost extremity ofthe cutout 142 is planar, corresponding to a planar face of the mandrel14 which borders the cutout 142. A central longitudinal axis (not shown)of the cutout 142 is aligned directly over a central longitudinal axis(not shown) of the notch 141. The cutout 142 does not communicate withthe notch 141. The tube 16 is hollow and cylindrical, and is made ofhighly heat conductive metal, such as copper, etc. An inner diameter ofthe tube 16 is generally equal to an outer diameter of the mandrel 14.

The first wick structure preform 15 is obtained from an elongated steelsheet with a plurality of pores. The elongated steel sheet is folded onitself repeatedly, and then inserted into the tube 16. A transversecross section of the first wick structure preform 15 is arch-shaped oncethe first wick structure preform 15 is received in the mandrel 14. Inparticular, an outer curvature of the first wick structure preform 15substantially matches an outer curvature of the mandrel 14, and an innercurvature of the first wick structure preform 15 substantially matchesan inner curvature of the mandrel 14 in the notch 141.

The first wick structure preform 15 is horizontally inserted into thenotch 141 of the mandrel 14. Then the mandrel 14 with the first wickstructure preform 15 is inserted into the tube 16. An amount of metalpowder is filled into the cutout 142 of the mandrel 14 in the tube 16.The tube 16 is vibrated until the metal powder is evenly distributedalong the length of the tube 16 in accordance with its particle size. Inparticular, smaller particles of the metal powder migrate to a lower endof the cutout 142 in the tube 16, and larger particles of the metalpowder migrate to an upper end of the cutout 142 in the tube 16. Thetube 16 with the mandrel 14, the metal powder and the first wickstructure preform 15 is heated at high temperature until the metalpowder sinters to form a second wick structure preform 17. In thisprocess, a bottom of the first wick structure preform 15 becomes joinedto the tube 16. A transverse cross section of the second wick structurepreform 17 is in the shape of a segment on a chord. In particular, thetransverse cross section includes a straight line 171 and an arcuateline 172 connecting the straight line 171. The arcuate line 172represents the part of the second wick structure preform 17 which isattached to the inner surface of the tube 16.

Referring to FIG. 6, the mandrel 14 is then drawn out of the tube 16,with the first and second wick structure preforms 15, 17 being retainedin the tube 16. The second wick structure preform 17 is at a top of theinner surface of the tube 16, and the first wick structure preform 15 isat a bottom of the inner surface of the tube 16, with the first andsecond wick structure preforms 15, 17 facing each other. Subsequentprocesses such as injecting a working medium into the tube 16, andevacuating and sealing the tube 16, can be performed using conventionalmethods. Thereby, a straight, circular heat pipe 18 is attained.Finally, the straight, circular heat pipe 18 is flattened. In thisprocess, the first and second wick structure preforms 15, 17 movedirectly toward each other, with the first wick structure preform 15deforming into a flattened structure under pulling force applied by thetube 16, and the second wick structure preform 17 deforming into agenerally triangular-prism shaped structure under pulling force appliedby the tube 16, until the second wick structure preform 17 firmlypresses the first wick structure preform 15. Thus, the flat heat pipe 10as illustrated in FIGS. 1 and 2 is formed. That is, the flattened tube16 forms the casing 11, the bent second wick structure preform 17 formsthe tapered second wick structure 13, and the bent first wick structurepreform 15 forms the flattened first wick structure 12.

Advantages of the method include the following. The cutout 142 of themandrel 14 has a planar inmost extremity. Thus, the cutout 142 can beeasily formed by directly milling the mandrel 14 using a milling machine(not shown). This reduces the cost of manufacturing the flat heat pipe10.

Referring to FIG. 7, a flat heat pipe 20 in accordance with a secondembodiment of the disclosure is shown. The flat heat pipe 20 differsfrom the flat heat pipe 10 of the first embodiment only in that a firstwick structure 22 is offset from a middle of a second wick structure 23,rather than being aligned directly beneath the middle of the second wickstructure 23. The first wick structure 22 is disposed in a middle of thecasing 11, but closer to the left side plate 113 of the casing 11 thanthe right side plate 114 of the casing 11. A bottom end of the secondwick structure 23 not in contact with the top plate 111 of the casing 11is snugly attached to a right lateral side of the top surface of thefirst wick structure 22. Alternatively, the first wick structure 22 canbe disposed in the middle of the casing 11 but closer to the right sideplate 114 of the casing than the left side plate 113 of the casing 11.In such case, the bottom end of the second wick structure 23 is snuglyattached to a left lateral side of the top surface of the first wickstructure 22.

During manufacture of the flat heat pipe 20, the first wick structurepreform 15 obliquely faces the second wick structure preform 17 (ratherthan directly facing the second wick structure preform 17 as isillustrated in FIG. 6). Then the circular flat heat pipe 18 isflattened.

Referring to FIG. 8, a flat heat pipe 30 in accordance with a thirdembodiment of the disclosure is shown. The flat heat pipe 30 differsfrom the flat heat pipe 10 of the first embodiment only in that a secondwick structure 33 is generally cuboid. A top surface of the second wickstructure 33 is attached to an inner surface of the top plate 111 of thecasing 11. In the illustrated embodiment, the second wick structure 33is located approximately at a middle of the inner surface of the topplate 111. A bottom surface of the second wick structure 33 snuglycontacts a top surface of a first wick structure 32.

Referring to FIGS. 9 and 10, aspects of an exemplary method formanufacturing the flat heat pipe 30 are illustrated. This method differsfrom the method summarized and illustrated in FIGS. 3 to 6 only in thata cutout 142 a of a mandrel 14 a defines a generally arcuate crosssection. A corresponding second wick structure preform 17 a in acircular flat heat pipe 18 a also has a generally arcuate cross section.The second wick structure preform 17 a, when flattened, forms the cuboidsecond wick structure 33.

Referring to FIG. 11, a flat heat pipe 40 in accordance with a fourthembodiment of the disclosure is shown. The flat heat pipe 40 differsfrom the flat heat pipe 30 of the third embodiment only in that a firstwick structure 42 and a second wick structure 43 are offset with respectto each other. In the illustrated embodiment, the second wick structure43 is located approximately at a middle of the inner surface of the topplate 111 of the casing 11, but closer to the right side plate 114 ofthe casing 11 than the left side plate 113 of the casing 11. The firstwick structure 42 is located approximately at a middle of the bottomplate 112 of the casing 11, but closer to the left side plate 113 thanthe right side plate 114 of the casing 11. A left side of the bottomsurface of the second wick structure 43 not in contact with the topplate 111 of the casing 11 is snugly attached to a right side of the topsurface of the first wick structure 42 not in contact with the bottomplate 112 of the casing 11. Alternatively, the second wick structure 43can be disposed approximately at the middle of the top plate 111 of thecasing 11 but closer to the left side plate 113, and the first wickstructure 42 can be disposed approximately at the middle of the bottomplate 112 of the casing 11 but closer to the right side plate 114. Insuch case, a right side of the bottom surface of the second wickstructure 43 is snugly attached to a left side of the top surface of thefirst wick structure 42.

During manufacture of the flat heat pipe 40, the first wick structurepreform 15 obliquely faces the second wick structure preform 17 a(rather than directly facing the second wick structure preform 17 a asis illustrated in FIG. 9). Then the circular flat heat pipe 18 a isflattened.

It is to be further understood that even though numerous characteristicsand advantages of the present embodiments have been set forth in theforegoing description, together with details of the structures andfunctions of the embodiments, the disclosure is illustrative only, andchanges may be made in detail, especially in matters of shape, size, andarrangement of parts within the principles of the invention to the fullextent indicated by the broad general meaning of the terms in which theappended claims are expressed.

1. A flat heat pipe comprising: a hollow, flattened casing, comprising atop plate and a bottom plate opposite to the top plate; and a first wickstructure and a second wick structure received in the casing, the firstwick structure being made of a folded steel sheet with a plurality ofpores, the second wick structure being made of sintered metal powder,the first and second wick structures disposed at inner surfaces of thebottom and top plates of the casing, respectively, the first and secondwick structures contacting each other, the casing defining two vaporchannels at opposite lateral sides of the combined first and second wickstructures, respectively.
 2. The flat heat pipe of claim 1, wherein abottom of the first wick structure is attached to the bottom plate, atop of the second wick structure is attached to the top plate, and a topof the first wick structure not in contact with the bottom plate isattached to a bottom of the second wick structure not in contact withthe top plate.
 3. The flat heat pipe of claim 1, wherein the first wickstructure is aligned with the second wick structure, and the second wickstructure is attached to a middle of the first wick structure.
 4. Theflat heat pipe of claim 3, wherein the second wick structure tapers fromthe top thereof farthest away from the first wick structure toward thebottom thereof in contact with the first wick structure, and the bottomof the second wick structure in contact with the first wick structureforms a rounded ridge attached to the middle of the first wickstructure.
 5. The flat heat pipe of claim 3, wherein the bottom of thesecond wick structure is attached to the top of the first wickstructure, and the second wick structure is generally cuboid.
 6. Theflat heat pipe of claim 1, wherein a center of the first wick structureand a center of the second wick structure are offset from each other aswould be viewed in transverse cross section of the casing.
 7. The flatheat pipe of claim 6, wherein the second wick structure tapers from atop thereof farthest away from the first wick structure toward a bottomthereof attached to the first wick structure.
 8. The flat heat pipe ofclaim 6, wherein one side of the second wick structure is attached tothe first wick structure, and the second wick structure is generallycuboid.
 9. The flat heat pipe of claim 1, wherein the second wickstructure is substantially triangular prism-shaped or cuboid, and abottom of the second wick structure not in contact with the casing isattached to the first wick structure.
 10. The flat heat pipe of claim 1,wherein the first wick structure is a flattened structure.
 11. A methodfor manufacturing a flat heat pipe, the method comprising: providing acylindrical mandrel, a hollow cylindrical tube and a steel sheet with aplurality of pores, the mandrel defining an elongated notch and anelongated cutout in a circumferential surface thereof, the notch and thecutout located opposite each other across a center axis of the mandrel,and an inner diameter of the tube being substantially equal to an outerdiameter of the mandrel; folding the steel sheet to form a first wickstructure; inserting the mandrel and the first wick structure into thetube, wherein the first wick structure is received in the notch of themandrel; filling an amount of metal powder into the cutout of themandrel in the tube, and sintering the metal powder to form a secondwick structure; drawing the mandrel out of the tube, wherein the firstand second wick structures remain attached to portions of an innersurface of the tube, and face each other; injecting a working mediuminto the tube, and evacuating and sealing the tube; and flattening thetube until the first wick structure becomes flattened and the secondwick structure contacts the first wick structure, thus forming a flatheat pipe, wherein the flat heat pipe defines two vapor channels atopposite lateral sides of the combined first and second wick structures,respectively.
 12. The method for manufacturing a flat heat pipe of claim11, wherein the cutout defines a generally arcuate cross section, andafter the mandrel is drawn out of the tube, the second wick structurehas a generally arcuate cross section.
 13. The method for manufacturinga flat heat pipe of claim 12, wherein after the tube is flattened, thesecond wick structure is generally cuboid.
 14. The method formanufacturing a flat heat pipe of claim 11, wherein an inmost extremityof the cutout is planar, and after the mandrel is drawn out of the tube,a transverse cross section of the second wick structure comprises astraight line and an arcuate line connecting the straight line, with thearcuate line corresponding to a portion of the second wick structureattached to the inner surface of the tube.
 15. The method formanufacturing a flat heat pipe of claim 14, wherein after the tube isflattened, the second wick structure generally tapers from one sidethereof farthest away from the first wick structure toward another sidethereof in contact with the first wick structure.
 16. The method formanufacturing a flat heat pipe of claim 11, wherein the notch defines anarcuate cross section, and the first wick structure defines an arcuatecross section before the tube is flattened.
 17. The method formanufacturing a flat heat pipe of claim 11, wherein the first wickstructure is pulled by the tube into a flattened shape during theflattening of the tube.
 18. The method for manufacturing a flat heatpipe of claim 11, wherein the first wick structure directly faces orobliquely faces the second wick structure before the tube is flattened.19. A flat heat pipe comprising: a hollow, flattened casing, comprisinga top plate and a bottom plate opposite to the top plate; and a firstwick structure and a second wick structure attached to inner sides ofthe bottom and top plates of the casing, respectively, the first wickstructure comprising a folded steel sheet with a plurality of pores, thesecond wick structure comprising sintered metal powder, the first andsecond wick structures snugly contacting each other, the casing definingtwo separate vapor channels at opposite lateral sides of the combinedfirst and second wick structures, respectively; wherein the flat heatpipe has an evaporator section and a condenser section respectivelylocated at opposite ends thereof along a longitudinal direction thereof,and the first wick structure extends longitudinally through theevaporator section and the condenser section.
 20. The flat heat pipe ofclaim 19, wherein the first wick structure obliquely faces or is alignedwith the second wick structure.